For the beef filling, fry the mince in a frying pan over a medium-high heat for 5-6 minutes, stirring occasionally and breaking it up with a wooden spoon, or until browned. Add the onion, mushrooms if using, green pepper and garlic and cook, stirring occasionally, for five minutes or until the vegetables are softened. Stir in the paprika and cumin and season to taste with salt and freshly ground black pepper. Pour in the beef stock and tomato purée and mix well. Cover and simmer gently for 10 minutes, stirring occasionally.
To serve, place the taco shells on a baking tray and warm them in the oven for 2-3 minutes. Spoon the beef filling into the warm taco shells, spoon over the guacamole and salsa and garnish with cucumber and spring onions.